In the automotive field, or where there is a need for charging an empty battery from a charged one, a pair of cables, commonly known as booster cables, are used. These cables are provided with two pairs of clamps for connection from one battery to the other. In making the connection however, there is a chance that an inexperienced person may connect the batteries with the wrong polarity and cause damage either to the batteries, to the cables or to his person because of excessive current passing through the cables.
In the prior art and, in particular, in U.S. Pat. No. 4,400,658 to Yates, the problem is stated but the solutions provided are not practical. In the device disclosed in the above patent, there is no provision for sensing the polarity of a very empty battery having only millivolts of potential across its terminals. Also, three position polar relays are described in the above patent that are impractical because such relays cannot be made to switch heavy currents which is a requirement for this application. Furthermore, in the device disclosed in the above patent, there is no provision for automatic disconnect of the cables after removal from the battery being charged and no monitoring and alarm circuits.